The present invention relates generally to a braking pressure control device which regulates the output fluid pressure for the wheel cylinders of the motor vehicle to increase at the same rate as the input fluid pressure from the master cylinder when the input fluid pressure is below a critical fluid pressure. The regulation provides an increase at a rate smaller than the rate of increase in the input fluid pressure when the input fluid pressure is above the critical fluid pressure. Further, the critical fluid pressure is varied in accordance with variation in the weight of the motor vehicle. More particularly, the invention relates to an improved braking pressure control device of this type including barrier means for preventing the flow of the input fluid pressure from striking the inertia responsive movable ball valve and exerting thereon a thrust or dynamic pressure which would move the valve to the valve seat to close the port thereof. The barrier means causes the input fluid pressure to flow through the surroundings of the valve to the valve seat.
As is well known in the art, usual motor vehicle hydraulic braking systems are such that the brakes are applied to the front and rear wheels concurrently. In this instance, if an excess amount of braking force is applied to the front wheels, the front wheels are locked earlier than the rear wheels to make it impossible for the driver to handle the motor vehicle. On the contrary, if the rear wheels are braked excessively, they are locked prior to the front wheels to cause the rear portion of the motor vehicle to swing transversely to the longitudinal direction thereof. Accordingly, in order to assure and increase the safety and stability of the vehicle during the braking operation, it is necessary to effect the distribution of the braking forces to lock the front and rear wheels concurrently.
When the vehicle is braked, the so-called nosedive phenomenon takes place in which the vehicle weight applied on the front wheels increases and the vehicle weight applied on the rear wheels decreases. Accordingly, it is necessary for concurrently locking the front and rear wheels to distribute to the front wheels a braking force greater than a braking force distributed to the rear wheels. It is also necessary that the distribution of the braking forces to the front and rear wheels is varied in accordance with variation in the vehicle weight. Thus, ideal characteristics of distribution of the braking forces to the front and rear wheels, when are illustrated on oblique coordinates having the axes of abscissa and ordinate indicating respectively thereon the ratios (deceleration rate ratios) Bf/W and Br/W of the braking forces Bf and Br on the front and rear wheels versus the vehicle weight W, are expressed by curves having tangents the angles of inclination of which are relatively large within a range of the origin to a certain value and are relatively small outside the range. Furthermore, the heavier the vehicle weight is, the higher the ideal characteristics curve is located on the coordinates.
It is accordingly necessary for providing the distribution of the braking forces which is close to the ideal characteristics curve to feed to the rear wheel cylinders a fluid pressure increasing at a rate smaller than that of increase in a fluid pressure fed to the front wheel cylinders or at a rate of zero when the fluid pressure fed to the front wheel cylinders exceeds a predetermined or critical fluid pressure. As an expedient for solving the problem, a limiting valve, proportioning valve or G valve was disposed in a rear braking circuit leading to the rear wheel cylinders. The limiting valve generates an output fluid pressure increasing at a rate of zero when an input fluid pressure exceeds a critical fluid pressure. The proportioning valve generates an output fluid pressure increasing at a rate lower than that of increase in an input fluid pressure when the input fluid pressure exceeds a critical fluid pressure. The G valve generates an output fluid pressure increasing at a rate less than an input fluid pressure when a predetermined rate of deceleration is attained. However, the output fluid pressure generated by these valves merely carried out the distribution of the braking forces approximating to a single ideal characteristics curve which accordingly, corresponds to a predetermined vehicle weight and, when the vehicle weight is varied, effected a distribution of the braking forces which largely deviated from an ideal characteristics curve corresponding to the vehicle weight varied.
On the other hand, most motor vehicles are in recent years provided with a hydraulic braking system of the tandem type which comprises front and rear braking circuits leading respectively separately to the front and rear wheel cylinders. However, a braking pressure control valve which is disposed in the rear braking circuit generated the same output fluid pressure as in the event of no failure of the fluid pressure in the front braking circuit in the event of the failure of the fluid pressure in the front braking circuit and as a result caused the deficiency of the braking force.
Thus, the applicants have proposed a braking pressure control device comprising a fluid pressure regulating valve serving as a proportioning valve or limiting valve, biasing means which urges the fluid pressure regulating valve and to which the master cylinder fluid pressure is fed to control the force of the biasing means, and an inertia responsive movable ball valve responsive to a predetermined rate of deceleration. The ball valve closes the path of flow of the fluid pressure to the biasing means to maintain the fluid pressure having been fed to the biasing means at a predetermined value. Consequently, the critical fluid pressure is varied in accordance with variation in the vehicle weight to generate the output fluid pressure which provides the distributions of the braking forces which approximate to the ideal characteristics curves corresponding to the vehicle weight varied. The fluid pressure regulating valve is also biased by the fluid pressure in the front braking circuit in a direction opposite to the biasing direction by the biasing means. Thus, in the event of the failure of the fluid pressure in the front braking circuit, the critical fluid pressure is sufficiently increased to generate the output fluid pressure high to compensate the deficiency of the braking force.
However, the conventional braking pressure control device has had a drawback in that support means for supporting the ball valve is formed through its central end portion with an aperture for passing the master cylinder fluid pressure to the biasing means. Therefore, the flow of the fluid pressure strikes the ball valve to exert a thrust thereon. The thrust moves the ball valve into a position to close an inlet port to the biasing means to hinder the braking pressure control device from exhibiting its desired function completely.